Thin film materials have heretofore been deposited on optical lens surfaces in combinations of layers that serve to reduce the reflection of light incident on that surface or to enhance the reflection of specific wavelengths. The system of thin films comprises layers of low refractive index which are alternated with layers of high refractive index. Occasionally, layers of refractive index intermediate between the high-index and low-index materials are employed. As can be appreciated, the coating layer materials must be transparent to the energy and not absorb in the spectrum of interest. Further, the coating material must be mechanically durable to external abrasive or erosive forces. They must be in a state of low internal strain to permit adhesion and cohesion when subjected to temperature excursions. A prime requirement of coating materials is that they be easily and consistently evaporateable with standard vacuum deposition equipment.
A common coating design used heretofore consisted of up to three different layer materials to provide anti-reflection to the window or lens upon which the coating is deposited. One component of this design is required to have a refractive index in the range of 1.6 to 1.4, depending on the wavelength region for which the anti-reflection coating is intended. This relatively low index value is often satisfied by fluoride compounds of the light metals or the rare earths. Examples are magnesium fluoride for the visible region, and thorium fluoride for the infrared region between 8 and 12 micrometers wavelength.
Thorium fluoride is currently the only low index, mechanically stable material known for infrared coating applications because thorium fluoride films are free of absorption in the range of from about 200 m.mu. up to about 3.mu.. A detrimental property of thorium fluoride is its radioactivity. The alpha radiation poses a sever health hazard should thorium fluoride dust be inhaled. Consequently, special handling and strict disposal problems are created whenever thorium fluoride is used. Furthermore, the radiation emitted from a coating of thorium fluoride can increase the noise background of photon detectors operating in proximity to the coated optic. Low index, transparent coating materials which can be deposited with low stress are needed to replace thorium fluoride. Examples of some potential materials are the rare earth fluorides, including cerous fluoride (CeF.sub.3); lanthanum fluoride (LaF.sub.3) and yttrium fluoride (YF.sub.3); aluminum fluoride (AlF.sub.3); barium fluoride and others.
Fluoride film layers must be deposited on substrates which are preheated to a temperature of above 200.degree. C. in order for the layers to grow with high density, and therefore provide good mechanical integrity. However, when the fluoride film coated substrates are cooled to room temperature, they exhibit high levels of tensile stress, often high enough to disrupt the film into cracks. Pure forms of fluoride compounds in the thick layer forms required for infrared coatings generally crack due to internal stresses. Some fluoride-compound films crack upon exposure to the moisture in the ambient air.
Pulker and Maser (Thin Solid Films, 59 (1979) 65) reported that the addition of a few percent calcium fluoride to magnesium fluoride resulted in films having reduced intrinsic stress. Pellicori (Thin Solid Films, 113 (1984) 287) demonstrated that mixtures of 10% to 20% barium fluoride in cerous fluoride reduced the stress to the degree that it was possible to deposit films of this mixture having a thickness which was twice as large as that previously obtainable with pure cerous fluoride. Pellicori's deposited films could be boiled in salt water with little degradation, while films of pure cerous fluoride are destroyed by such exposure.
There is electron micrographic evidence that the microstructure of the mixed film layer is amorphous rather than the columnar form typical for fluoride layers. This glassy microstructure is denser and less permeable to water absorption so that the film layers exhibit greater stability in their optical and mechanical properties when exposed to the normal atmosphere.
The experiments referenced above were done using mixtures of fine powers of the respective compounds. However, the vacuum evaporation of such materials in powdered form is difficult because of the inevitable occurrence of outgassing of absorbed water and other gasses from the powder grains. Similarly, there is a spattering of particles found on the coated optic due to the spitting of the powders when heated. In addition, consistency in evaporation rate and deposited composition is difficult to achieve with powder mixtures. Attempts have been made to simultaneously evaporate two materials from discrete sources in the correct proportion. However, this technique requires the expensive duplication of source and control equipment, and a consistent mixing ratio is difficult to maintain. For these reasons, the apparent advantages afforded by mixtures could not be realized in coating production.
Until now, no infrared-transparent, low-refractive index material has been available which could be deposited reproducibly to form durable, low-stress, hazard-free film layers having a thickness greater than about 1000 nm with a refractive index near 1.4 to 1.6 and with very little or no absorption.
Thus a need still exists for a non-radioactive material which can be used to coat lenses for use in laser transmissions and the like and for methods of producing and applying such coatings to a lens to provide a uniform stress-free coating having a thickness of at least from about 800 to about 1800 nanometers. It is toward the definition and solution of these needs that the present invention is directed.